Microanatomy of the central myelin portion and transitional zone of the oculomotor and abducens nerves

Background: The microanatomy of the central myelin portion and transitional zone of several cranial nerves including trigeminal, facial, vestibulocochlear, glossopharyngeal, and vagus nerves have been clearly demonstrated to provide information for neurovascular compression syndrome such as trigeminal neuralgia and hemifacial spasm. However, the study of oculomotor and abducens nerve is limited. Material and methods: Oculomotor and abducens nerves were harvested with a portion of brainstem and embeded in paraffin. Longitudinal and serial sections from ten of each cranial nerve were stained and a photomicrograph was taken to make the following observations and measurements: 1) patterns of central myelin portion, 2) length of central myelin portion, and 3) depth of central myelin-peripheral myelin transitional zone. Results: For oculomotor nerve, the longest central myelin bundle was always seen on the first nerve bundle and that the length of central myelin decreased gradually. For abducens nerve, morphological patterns were classified into 4 types based on number of nerve rootlets emerging from the brainstem and number of nerve bundles in each rootlet. Length of central myelin portion was between 0.36-6.10 mm (2.75+0.83 mm) and 0.13-5.01 mm (1.66+1.39 mm) for oculomotor and abducens nerves, respectively. The oculomotor nerve transitional zone depth was 0.07-058 mm (0.23+0.07 mm), while for abducens nerve, depth was 0.05-0.40mm (0.16+0.07 mm). Positive weak correlations between central myelin and depth of TZ were found in oculomotor nerve (r +0.310, p<0.05) and abducens nerves (r +0.413, p<0.05). Conclusions: Detailed microanatomy of the central myelin and transitional zone might be beneficial for locating the site of compression in neurovascular conflicts at oculomotor and abducens nerves